Revision as of 12:14, 17 November 2017

cron is the time-based job scheduler in Unix-like computer operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration.

Configuration

Activation and autostart

After installation, the daemon will not be enabled by default. The installed package likely provides a service, which can be controlled by systemctl. For example, cronie uses cronie.service.

Check /etc/cron.daily/ and similar directories to see which jobs are present. Activating cron service will trigger all of them.

Note:cronie provides the 0anacronhourly job, which allows for delayed runs of other jobs e.g. if the computer was switched off at the moment of standard execution.

Handling errors of jobs

cron registers the output from stdout and stderr and attempts to send it as email to the user's spools via the sendmail command. Cronie disables mail output if /usr/bin/sendmail is not found. In order for mail to be written to a user's spool, there must be an smtp daemon running on the system, e.g. opensmtpd. Otherwise, you can install a package that provides the sendmail command, and configure it to send mail to a remote mail exchanger. You can also log the messages by using the -m option and writing a custom script.

Example with ssmtp

ssmtp is a send-only sendmail emulator which delivers email from a local computer to an smtp server. While there are currently no active maintainers, it is still by far the simplest way to transfer mail to a configured mailhub. There are no daemons to run, and configuration can be as simple as editing 3 lines in a single configuration file (if your host is trusted to relay unauthenticated email through your mailhub). ssmtp does not receive mail, expand aliases, or manage a queue.

Install ssmtp, which creates a symbolic link from /usr/bin/sendmail to /usr/bin/ssmtp. You must then edit /etc/ssmtp/ssmtp.conf. See ssmtp for details. Creating a symbolic link to /usr/bin/sendmail insures that programs like S-nail (or any package which provides /usr/bin/mail will just work without modification.

Restart cronie to insure that it detects that you now have a /usr/bin/sendmail installed.

Example with msmtp

Install msmtp-mta, which creates a symbolic link from /usr/bin/sendmail to /usr/bin/msmtp. Restart cronie to make sure it detects the new sendmail command. You must then provide a way for msmtp to convert your username into an email address.

user can check his/her mail in with any reader able to handle mbox format, or just have a look at the file /var/spool/mail/user. If everything goes as expected, you can enable opensmtpd for future boots.

This approach has the advantage of not sending local cron notifications to a remote server. On the downside, you need a new daemon running.

Note:

At the moment of writing the Arch opensmtpd package does not create all needed directories under /var/spool/smtpd, but the daemon will warn about that specifying the required ownerships and permissions. Just create them as suggested.

Even though the suggested configuration does not accept remote connections, it's a healthy precaution to add an additional layer of security blocking port 25 with iptables or similar.

Long cron job

Suppose this program is invoked by cron :

#!/bin/sh
echo "I had a recoverable error!"
sleep 1h

What happens is this:

cron runs the script

as soon as cron sees some output, it runs your MTA, and provides it with the headers. It leaves the pipe open, because the job hasn't finished and there might be more output.

the MTA opens the connection to postfix and leaves that connection open while it waits for the rest of the body.

postfix closes the idle connection after less than an hour and you get an error like this :

To solve this problem you can use the command chronic or sponge from moreutils.
From their respective man page:

chronic

chronic runs a command, and arranges for its standard out and standard error to only be displayed if the command fails (exits nonzero or crashes). If the command succeeds, any extraneous output will be hidden.

sponge

sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file… If no output file is specified, sponge outputs to stdout.

Spaces are used to separate fields. To fine-tune your schedule you may also use one of the following symbols:

Symbol

Description

*

Wildcard, specifies every possible time interval

,

List multiple values separated by a comma.

-

Specify a range between two numbers, separated by a hyphen

/

Specify a periodicity/frequency using a slash

For example, the line:

*/5 9-16 * 1-5,9-12 1-5 ~/bin/i_love_cron.sh

Will execute the script i_love_cron.sh at five minute intervals from 9 AM to 4:55 PM on weekdays except during the summer months (June, July, and August). More examples and advanced configuration techniques can be found below.

Besides, crontab has some special keywords:

@reboot at startup
@yearly once a year
@annually ( == @yearly)
@monthly once a month
@weekly once a week
@daily once a day
@midnight ( == @daily)
@hourly once an hour

Basic commands

Crontabs should never be edited directly; instead, users should use the crontab program to work with their crontabs. To be granted access to this command, user must be a member of the users group (see the gpasswd command).

Default editor

To use an alternate default editor, define the EDITOR environment variable in a shell initialization script as described in Environment variables.

As a regular user, su will need to be used instead of sudo for the environment variable to be pulled correctly:

$ su -c "crontab -e"

To have an alias to this printf is required to carry the arbitrary string because su launches in a new shell:

alias scron="su -c $(printf "%q " "crontab -e")"

Running X.org server-based applications

Cron does not run under the X.org server therefore it cannot know the environmental variable necessary to be able to start an X.org server application so they will have to be defined. One can use a program like xuserrun-gitAUR to do it:

17 02 * ... /usr/bin/xuserrun /usr/bin/xclock

Or they can be defined manually (echo $DISPLAY will give the current DISPLAY value):

17 02 * ... env DISPLAY=:0 /usr/bin/xclock

If running notify-send for desktop notifications in cron, notify-send is sending values to dbus. So it needs to tell dbus to connect to the right bus.
The address can be found by examining DBUS_SESSION_BUS_ADDRESS environment variable and setting it to the same value. Therefore:

Asynchronous job processing

Cronie

Cronie contains the standard UNIX daemon crond that runs specified programs at scheduled times and related tools.
It is based on the original cron and has security and configuration enhancements like the ability to use pam and SELinux.

Dcron

Vanilla dcronAUR supports asynchronous job processing. Just put it with @hourly, @daily, @weekly or @monthly with a jobname, like this:

@hourly ID=greatest_ever_job echo This job is very useful.

Cronwhip

cronwhipAUR is a script to automatically run missed cron jobs; it works with the former default cron implementation, dcron.
See also the forum thread.

Anacron

Anacron is a full replacement for dcron which processes jobs asynchronously.

It is provided by cronie. The configuration file is /etc/anacrontab. Information on the format can be found in the anacrontab(5)man page. Running anacron -T will test /etc/anacrontab for validity.

Fcron

Like anacron, fcron assumes the computer is not always running and, unlike anacron, it can schedule events at intervals shorter than a single day which may be useful for systems which suspend/hibernate regularly (such as a laptop). Like cronwhip, fcron can run jobs that should have been run during the computer's downtime.

When replacing cronie with fcron be aware the spool directory is /var/spool/fcron and the fcrontab command is used instead of crontab to edit the user crontabs. These crontabs are stored in a binary format with the text version next to them as foo.orig in the spool directory. Any scripts which manually edit user crontabs may need to be adjusted due to this difference in behavior.

A quick scriptlet which may aide in converting traditional user crontabs to fcron format:

Ensuring exclusivity

If you run potentially long-running jobs (e.g., a backup might all of a sudden run for a long time, because of many changes or a particular slow network connection), then flock (util-linux) can ensure that the cron job won't start a second time.

Cronie provides both cron and anacron functionalities: cron runs jobs at regular time intervals (down to a granularity of one minute) as long as the system is available at the specified time, while anacron executes commands at
intervals specified in days. Unlike cron, it does not assume that the system is running continuously. Whenever the system is up, anacron checks if there are any jobs that should have been run and handles them accordingly.

A cron job can be defined in a crontab-like file in the /etc/cron.d directory or added within the /etc/crontab file. Note the latter is not present by default but is used if it exists. As instructed by /etc/cron.d/0hourly, any executable file in /etc/cron.hourly will be run every hour (by default at minute 1 of the hour). Files in /etc/cron.minutely are executed every minute if instructed accordingly in /etc/cron.d/0hourly. The executables are typically shell scripts, symlinks to executable files can also be used.
The /etc/cron.deny file includes the list of users not allowed to use crontab, without this file, only users listed in /etc/cron.allow can use it.

Anacron works similarly, by executing the files in the /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly directories, placed there depending on the desired job frequency. The cron job /etc/cron.hourly/0anacron makes sure that anacron is run once daily to perform its pending tasks.

Note: cronie uses run-parts to carry out scripts in the different directories. The filenames should not include any dot (.) since run-parts in its default mode will silently ignore them (see run-parts(8)). The names must consist only of upper and lower-case letters, digits, underscores and minus-hyphens.

Note: the output of systemctl status cronie might show a message such as CAN'T OPEN (/etc/crontab): No such file or directory. However, this can be ignored since cronie does not require one.

Tip: To prevent the sending of output and stop email alert, add >/dev/null 2>&1 at the end of the line for each cron job to redirect output to /dev/null.

0 1 5 10 * /path/to/script.sh >/dev/null 2>&1

You can also set MAILTO=”” variable in your crontab file to disable email alerts.

Dcron

The cron daemon parses a configuration file known as crontab. Each user on the system can maintain a separate crontab file to schedule commands individually. The root user's crontab is used to schedule system-wide tasks (though users may opt to use /etc/crontab or the /etc/cron.d directory, depending on which cron implementation they choose).